A spokesman for the mine said: "The police and mine inspectors and our senior managers are underground at the moment."

Earlier, Labour MP Tom Blenkinsop, who represents Middlesbrough South and East Cleveland, said: "There has been a serious incident believed to be a gas explosion and we are waiting for further details about what has happened."

Boulby is the deepest mine in Britain and potash production began in 1973Credit:
Ian Forsyth/Getty

His "thoughts and prayers" were with the workforce. There was an underground fire at the mine in April, which left seven workers affected by smoke and they were taken to hospital for checks.

Last year the company announced job losses in a "significant restructuring" of its operations, including moving from producing potash to Polysulphate fertiliser.

The mine is 1,400m deep and its tunnels go far out under the North Sea. Miners take seven minutes to travel down by lift and the temperature at the bottom can reach 104F (40C).

A Cleveland Police Spokeswoman said: “We can confirm we were called at around 5.20am to an incident at Boulby Potash during which a man has died. Police remain at the scene and we are working to understand the circumstances surrounding the incident. Our thoughts are with family, friends and colleagues.”